But that topic is locked so I could not reply there.
I'm running Windows XP SP3 and the icon was there until a few days ago. Windows update has installed stuff automatically but I have made no other changes to the machine that I am aware of.
The previous answer was as follows:
If the icon doesn't appear on the task bar it could be one of two things. Either the Microsoft Security Essentials application has failed to start, resulting in the icon being missing, or it could be that the icon is in fact there but is hidden.
If the icon is hidden, you can right click on the taskbar and choose Properties. Here you will see an option to "Hide Inactive Icons" which can be unchecked as a test, then click OK. This will display all the icons that are hidden or inactive; please check to see if you see the Microsoft Security Essentials icon appear.
If not, we can try to launch the application manually to see if it starts. Check the desktop for the Microsoft Security Essentials icon and try to launch it manually. If you receive an error message make sure to write it down. After launching it manually, check again for the icon in the taskbar.Lastly, the problem might be that Windows is not configured to launch the Microsoft Security Essentials application when you boot the computer. Click Start then Run, and type MSCONFIG and click OK. Here click on the Startup tab and ensure that Microsoft Security Essentials is listed here and it is checked.
In my situation, none of the above suggestions seem to apply, as
1. MsMpEng.exe is showing as running in Task Manager
2. The icon is not hidden. The box in the task bar preferences is NOT ticked. (I tried ticking it and then unticking it again - no change).
3. I can run the program again from the start menu - The resultant window looks perfectly normal and everything is shown as running fine. But there is still no icon visible in the tray when that window is there.
4. However, out of interest, I cannot see anything that looks like Security essentials or MsMpEng.exe in the Startup tab of MSConfig - As the exe [u]is[/u] running, has it been moved to a different place?